The present invention concerns a self-perforating (punching) and thread-forming connecting element, in particular a screw, with a screw shaft and screw tip, which tapers to a sharp point, as well as a single or multiple thread, which runs at least partly along the screw shaft and the tip of the screw up to the pointed end.
Such a connecting element is known from the DE 33 35 092 A1. These types of connecting elements, in particular screws, have the function of being screwed directly into soft materials, such as, above all, wood materials of all kinds, and specifically without requiring any prior drilling of a core hole. The screw is thus screwed directly into the material, during which process a material-displacement effect without the generation of chips is achieved. The screw, including the tip of the screw, is then seated in the material, so that the entire thread up to the tip participates in the retaining function and, therefore, high loosening moments and pull-out forces or moments can be achieved. In this case, the known screw, by virtue of a wave-shaped design of the edge of the thread, has a reduced screw-in moment, while the tip of the thread acts as a kind of rubbing tool due to the fact that the thread terminates in a pointed end. This screw has proved itself very well in practical use.
The DE 195 32 874 A1 describes a very similar connecting element, where additionally in the region of the tip at least one rubbing edge portion extends in the longitudinal direction of the screw along a part of the length of the tip of the screw. The rubbing region has a slight cutting effect.
The U.S. Pat. No. 3,246,556 describes a thread-forming, but not self-perforating screw for thin sheet metal materials, i.e. for threading a screw into a previously drilled core hole (pilot hole). For this purpose, this known screw consists of two sections with basically different designs and functions, and more specifically of a thread-forming section at the end or the tip and a retaining section following it. The retaining section consists of a cylindrical shaft with a circular and helical thread. The thread-forming section, on the other hand, has a cross-section, which consists of an equilateral triangle, while, in this region, the outer edge of the thread also has a corresponding triangular shape. The thread-forming section has the exclusive function of forming a thread in a pre-drilled core hole of a thin sheet metal material at a relatively low screw-in moment and without generating chips, in which hole subsequently only a portion of the thread of the retaining section is seated. The thread-forming section thus does not contribute to the retaining function, but the loosening and pull-out moment is exclusively determined by the retaining section.
The present invention is based on the task of creating a connecting element of the generic type, which is especially well suited for screwing it directly into soft materials, such as, for example, wood, plywood, composite materials and the like without requiring pre-drilling, and where a small tightening moment and a high loosening and pull-out moment is achieved.
According to the invention this is achieved by means of screw tips which, at least in a partial region of their axial length, have an essentially polygonal cross-section, while the thread is constructed as a helix-shaped raised portion with an outer edge of the thread, and where the edge of the thread--viewed in an axial projection--proceeds along the region of the screw tip in the form of a spiral as the radius becomes smaller, and in a circular form and with a constant radius in the region of the shaft of the screw. As far as the polygonal cross-section is concerned, it consists of a triangular, quadrangular or pentagonal cross-section, which in a preferred embodiment, contains in particular side surfaces having a convex curvature and/or, under certain circumstances, rounded corners. In this case, it is useful if the region having the essentially polygonal cross-section extends at least to the pointed end of the tip of the thread, and specifically with a surface area approaching zero at the tip.
By means of the design according to the invention, a generation of chips is practically avoided when the screw is screwed into position. Rather, the self-perforating effect of the connecting element according to the invention is based on the fact that the tip of the thread pushes itself into the material by virtue of its polygon-shaped core cross-section, and does so by means of achieving a good radial displacement effect due to the rising and falling moments occurring during the rotation. Yet, a high loosening moment (high retaining force) is achieved, because after the screw is screwed into position, the displaced soft material practically clings to the polygonal cross-section, so that in essence a form-fit or at least a press-fit is achieved. As a result of the fact that the edge of the thread proceeds in a continuous circular or spiral form, a very high pull-out moment or pull-out force is also ensured when the screw has been screwed into position because it, as well as the tip of the screw, is now seated in the material. This means that the connecting element according to the invention has practically only a retaining section, because the tip of the thread also participates in the retaining function, while nevertheless an advantageously low screw-in moment is ensured by virtue of the special design of the tip of the thread.
Further advantageous characteristics of the embodiment of the invention are contained in the dependent claims as well as in the description below.